/Engine/Source/Runtime/Engine/Classes/Engine/World.h
Check GameMode PreLogin results for split screen player joins.
void PreLoginCompleteSplit ( const [FString](API\Runtime\Core\Containers\FString) & ErrorMsg, [TWeakObjectPtr](API\Runtime\Core\UObject\TWeakObjectPtr)< [UNetConnection](API\Runtime\Engine\Engine\UNetConnection) > WeakConnection, [FUniqueNetIdRepl](API\Runtime\Engine\GameFramework\FUniqueNetIdRepl) SplitRequestUniqueIdRepl, [FString](API\Runtime\Core\Containers\FString) SplitRequestURL )